1. Technical Field
The present invention relates to a laser annealer and a laser thin-film forming apparatus. Particularly, the invention relates to a laser annealer using a GaN (gallium nitride) type semiconductor laser as a light source and a laser thin-film forming apparatus for depositing a thin-film on a substrate that is accommodated in a closed space using products obtained by applying a laser beam to a material gas or a solid material in the closed space and causing a decomposing or synthesizing reaction.
2. Description of the Related Art
From the viewpoint of reduction in the size, weight, and cost of flat panel displays such as liquid crystal displays (LCDs) and organic EL (electroluminescence) displays, nowadays much attention is paid to system-on-glass (SOG) TFT technology. In this technology, not only thin-film transistors (TFTs) for pixel display gates but also driver circuits, a signal processing circuit, an image processing circuit and others are formed directly on a glass substrate of an LCD.
Whereas conventionally amorphous silicon has been used for TFTs for pixel display gates, polysilicon which exhibits a high carrier mobility is necessary for SOG-TFTs. The deformation temperature of glass is as low as 600° C., and crystal growth techniques utilizing temperatures higher than 600° C. cannot be used for formation of a polysilicon film. A technique commonly used for formation of a polysilicon film is an excimer laser annealing (ELA) method. In this method, an amorphous silicon film is formed at a low temperature (100 to 300° C.), melted thermally by applying to it pulse laser light generated by an XeCl excimer laser (wavelength: 308 nm), and then crystallized by a cooling process. By using the ELA, a polysilicon film can be formed on a glass substrate without giving thermal damage on a glass substrate.
The optical output power of the XeCl excimer laser is unstable and the output light intensity varies in a range of ±10%. When it is applied to ELA, the crystal grain size of a polysilicon film varies and the reproducibility is low. When the XeCl excimer laser is applied to ELA, the repetition frequency of pulse driving of the XeCl excimer laser is as low as 300 Hz. Thus, forming continuous crystal grain boundaries is difficult and high carrier mobility cannot be obtained. Also annealing cannot be performed at high speed in a large area. Further, the XeCl excimer laser has problems unique to gas lasers. They include the lives of a laser tube and a laser gas are as short as about 1×107 shots and the maintenance cost will increase, the laser apparatus becomes larger, and the energy efficiency is as low as 3%.
In the ELA, to produce a polysilicon film that is uniform in crystal characteristics, a laser beam that is emitted from an excimer laser is shaped by a homogenizer optical system into a line-shaped beam having a flat-top intensity profile. The use of the XeCl excimer laser causes that the profile of a line-shaped beam is not uniform because the optical output power of the XeCl excimer laser is unstable and the output light intensity varies in a range of ±10%. The wavelength 308 nm of the XeCl excimer laser is in the ultraviolet range, and an optical system using a special material is necessary to produce a line-shaped beam.
An example in which a high-output-power solid-state laser is used instead of an excimer laser is disclosed by HARA et al. in Singakugiho (Technical Report of IEICE) ED2001–10, The Institute of Electronics, Information and Communication Engineers of Japan, pp. 21–27, 2001. With this example, amorphous silicon does not absorb light sufficiently and the heat generation efficiency is low at a low-power oscillation wavelength 532 nm.
The annealing rate is serious in realizing mass-production and enabling processing of a large-side substrate.
Incidentally, among film forming methods using a laser is a method in which a thin film is deposited on a substrate by utilizing a decomposing reaction that is caused by applying a laser beam to a material gas or a solid material.
A thin-film forming method by CVD (chemical vapor deposition) is such that a thin film is deposited on a substrate by inputting a laser beam to a chamber as a closed space and decomposing, by optical energy, a material gas that is supplied to the chamber. A thin-film forming method by laser sputtering is such that a thin film is deposited on a substrate by inputting a laser beam to a chamber as a closed space and vaporizing a solid material in the chamber by applying the laser beam to it.
As such, methods using laser light as excitation energy are advantageous over other PVD (such as evaporation and sputtering) and CVD (such as thermal and plasma) method in that the temperature of a substrate on which a film is to be formed does not increase. Hence a thin film with weak internal stress can be formed, which can reduce a warp of a large-size substrate in particular.
As shown in FIG. 45, a thin-film forming method is known, in which an ArF excimer laser 140 (wavelength: about 193 nm) is used.
In the ArF excimer laser 140, the pulse width is fixed and a grain diameter for attaining desired properties cannot be obtained. The repetition frequency of pulse driving of the ArF excimer laser 140 is as low as 2 kHz and energy accumulation takes time. Hence the film formation rate is low and the production efficiency is low.
Furthermore, the ArF excimer laser 140 is expensive and its life is as short as about 5×107 shots. Its running cost is high.
Further, in the conventional laser sputtering thin-film forming method as shown in FIG. 45, a thin film is formed as follows. A laser beam emitted from the single ArF excimer laser 140 is deflected in the width direction (direction A) of a substrate 94 by an optical scanning system 142. A stage 144 mounted with the substrate 94 is moved in the length direction (direction B). Thus, this apparatus cannot be used for thin-film formation for a product having a large film formation area such as Si-TFTs for a liquid crystal display, a solar cell, or the like.